Merge branch 'hoppingkind' into devel

doc/source/images/ab_ring.py.diff

doc/source/images/graphene.py.diff

doc/source/images/quantum_well.py.diff

doc/source/reference/kwant.builder.rst

doc/source/tutorial/ab_ring.py

doc/source/tutorial/closed_system.py

doc/source/tutorial/graphene.py

doc/source/tutorial/quantum_well.py

doc/source/tutorial/quantum_wire_revisited.py

doc/source/tutorial/spin_orbit.py

doc/source/tutorial/superconductor_transport.py

doc/source/tutorial/tutorial1.rst

doc/source/tutorial/tutorial2.rst

doc/source/tutorial/tutorial4.rst

doc/source/whatsnew/0.3.rst

kwant/builder.py

kwant/graph/tests/test_slicer.py

kwant/lattice.py

kwant/linalg/tests/test_mumps.py

kwant/solvers/tests/_test_sparse.py